letpe don's Wall

Physics is the only science that explicitly studies time, but even physicists agree that time is one of the most difficult properties of our universe to understand. Even in the most modern and complex physical models, though, time is usually considered to be an ontologically “basic” or primary concept, and not made up of, or dependent on, anything else.

In the sciences generally, time is usually defined by its measurement: it is simply what a clock reads. Physics in particular often requires extreme levels of precision in time measurement, which has led to the requirement that time be considered an infinitely divisible linear continuum, and not quantized (i.e. composed of discrete and indivisible units). With modern atomic time standards like TAI and UTC (see the section on Time Standards) and ultra-precise atomic clocks (see the section on Clocks), time can now be measured accurate to about 10−15 seconds, which corresponds to about 1 second error in approximately 30 million years.